Real harm would be hard to prove, but this is deeply unprofessional on Twitter's part. You could probably make a case for implied endorsement, and maybe even win, but that seems besides the point.
This would be like a Google commercial featuring "blowski, Google User" in one part. There's no real harm to you, and you are a Google user (I assume), but you've been publicized in a massive way that you never consented to. People do not appreciate being thrust into the limelight suddenly and without warning.
This isn't a huge outrage, but it does suggest some process failure on Twitter's part. You never, ever, ever, ever, ever mock up designs with real user data. This looks like what happened here - someone thoughtlessly used the username and profile pic in a mockup, which eventually made its way to PR without verification.
There are some poor processes in Twitter's design department, evidently. Hopefully they'll learn from this. Never mock up designs with real data. Twitter should have a reserve of handles/profiles they actually own for marketing uses.
